1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to devices and methods for generating seismic waves in an underground formation and, more particularly, to mechanisms and techniques for generating seismic waves with volumetric piezoelectric sources.
2. Discussion of the Background
Seismic wave sources may be used to generate seismic waves in underground formations for investigating the underground structure based on seismic images formed from reflections of the seismic waves at interfaces between formation layers that have different acoustic properties. The reflections are detected by seismic receivers. A seismic survey investigating the underground structure may be performed on land or water.
Focusing now on land seismic sources, in early such sources, a force was applied to the formation through a bell-shaped mass attached to the top of a pillar, the pillar being mounted on a metal plate resting directly on the formation or bolted to a concrete slab (i.e., a coupled reactive mass). This type of coupling was abandoned because of inadequate repeatability of the seismic waves obtained for a surface source, even when fixed on a concrete slab, partly due to variations in temperature and humidity in the weathering zone (WZ). Another reason for abandoning this arrangement was that while the energy delivered peaks around 80-100 Hz (depending on the mass), it decays abruptly at low frequencies.
A conventional seismic wave source 10 (described in U.S. Pat. No. 7,420,879 to Meynier et al.) is illustrated in FIG. 1. The source 10 includes plural vibrators (electromechanical, electromagnetic, hydraulic, piezoelectric, magnetostrictive, etc.) forming a pillar 1 in contact with plates 2 and 3. A force is applied to the formation by the pillar 1 via the plates 2 and 3, thereby generating a seismic wave.
The pillar 1, which is covered with a deformable membrane 4, is connected by a cable 5 to a signal generator 6. The source 10 is placed in a cavity or well W, for example, of 15 to 20 cm in diameter, at a desired depth under the WZ, usually between 20 and 200 m. A coupling material 7, such as cement or concrete, is injected into the well to be in direct contact with pillar 1 over the total length thereof and with the plates 2 and 3. In order to allow the coupling material 7 to be homogeneously distributed in the space between plates 2 and 3, the plates may have perforations 8. The diameter of plates 2 and 3 must correspond substantially to the diameter of the cavity or well W so as to achieve maximum coupling surface area.
The signal generator 6 generates an excitation signal in a frequency sweep or a single frequency, causing elements forming the pillar 1 to expand or contract temporarily along the pillar's longitudinal axis.
The metal plates 2 and 3 are mounted on the pillar ends to improve the coupling of the pillar 1 with the coupling material 7. The coupling material 7 intermediates the coupling between the source and the formation. For example, the plates 2 and 3 have a thickness of 10 cm and a diameter of 10 cm. The pillar 1 has a length exceeding 80 cm. The membrane 4 may be made of polyurethane and surround the pillar 1 to decouple it from the coupling material (cement) 7. Thus, only the end portions of the pillar 1 and the plates 2 and 3 are coupled with the coupling material (cement) 7. Upon receiving an excitation (electrical signal) from the signal generator 6, the source 10 generates forces along the pillar's longitudinal axis. This conventional source provides good repeatability and high reliability, once a good coupling is accomplished.
A typical pillar has a cylindrical shape with a radius of 5 cm and a length of 95 cm. This pillar may consist of 120 ceramics made, for example, of lead-zirconate-titanate (PZT) known under the commercial name NAVY type I. Each ceramic may have a ring shape with 20 mm internal diameter, 40 mm external diameter and 4 mm thickness. The maximum length expansion obtainable for this pillar in the absence of constraints is 120 μm, corresponding to a volume change of about 1000 mm3. The electrical signals fed to the pillars have 5-300 Hz, 2500 V peak maximum and 2 A peak maximum.
However, the conventional source has the disadvantage of producing a large amount of energy corresponding to transverse waves (i.e., S-waves, in which the displacement of the medium is perpendicular to the direction of wave propagation) relative to the energy corresponding to the more desirable longitudinal waves (i.e., P-waves, in which the displacement of the medium is parallel to the direction of wave propagation, and the P-waves propagation speed is nearly twice the S-waves propagation speed). Another disadvantage is related to the source's repeatability and reliability. It depends on the impedance adaptation between the source, the coupling material and the formation. Due to the strong impedance of the source and the low generated displacements, the best efficiency is obtained when the source is coupled in a hard environment with cement. Therefore, a conventional source is not necessarily suitable for very soft formations. Yet another disadvantage is that the radiation pattern of the conventional source (long pillar of ceramics) is adapted for vertical wells but not for horizontal wells.
Thus, there is a need to develop a seismic source capable of generating seismic waves carrying a larger fraction of the energy as P-waves than conventional sources, better adapted to soft formations and/or suitable for deployment in horizontal wells.